An intake for a turbofan gas turbine engine nacelle is required to supply the fan of the engine with appropriate quantities of favourably conditioned air with low pressure loss and distortion levels over the complete flight envelope. The nacelle intake may also be required to absorb noise generated by the gas turbine engine.
As shown in FIG. 1, the intake is formed by an inner wall of the nacelle and has, in flow series: a flared intake lip 1 designed to reduce off-design airflow separation by controlling the level of flow overspeeds, an intake throat 3, and a diffuser 5 to guide the airflow into the fan section. The diffuser also provides a settling length over which flow asymmetry is evened out.
The diffuser 5 typically ends at the engine face 7, which coincides with the upstream end of the fan case surrounding the fan section. On proceeding downstream, the air flow exiting the diffuser is thus bounded by structures, such as acoustic panels, which form an airwashed surface of the flow annulus formed within the fan case at the front of the fan section. In some engines such structures can be configured so that they further diffuse the incoming air to a position just upstream of the leading edges of the tips of the fan.
Older intake designs can have a conic diffuser with blend radii front and rear to connect to the lip and the airwashed surface at the front of the fan section. More recent intake designs can have diffusers constructed from CAD-generated splines which use the full diffuser length to progressively turn the airflow onto the engine axis.
At its downstream end, the diffuser 5 can be tangency matched to the, typically cylindrical, airwashed surface at the front of the fan section, for example using the approach described in EP A 2148064. Another option is to allow a degree of tangency mismatch (e.g. of up to 2°) which is constant around the circumference of this airwashed surface. In general, control of the diffuser curvature before this point is either made equal to one fan radius all around the circumference of the intake or is not directly controlled and subject to other intake design parameters.
To reduce nacelle drag, the overall nacelle size should be as small as possible. However, due to external cowl design considerations, this is generally achieved by having the lip region 1 offset vertically from engine centreline X-X and having the lip centre axis Y-Y and engine centerline angled to each other. This results in a three dimensional diffuser shape with higher levels of flow diffusion towards the bottom line than the top line.
In particular, the three dimensionality of the intake diffuser shape resulting from the lip region vertical offset, results in a non-uniform diffusion rate and hence can cause circumferential variation in airflow velocity delivered to the fan. This in turn can lead to a non-uniform aero loading on the fan blades as they rotate, changing the local inflow velocity and hence the local aerodynamic forces on the blade. This can produce a lower level of fan efficiency than desired, and higher stress on the fan blades which can reduce blade life. To compensate the fan outlet guide vanes (OGV's) located behind the fan can be cambered and staggered to counteract the intake flow asymmetry, but this increases complexity and cost. In addition configuring the OGV's to counteract intake flow asymmetry can reduce fan module aerodynamic efficiency.
High levels of flow asymmetry through the intake diffuser section can also adversely affect the level of attenuation to which acoustic liners in the intake can suppress fan buzz saw noise.